OVEREXPRESSION OF XMYOD OR XMYF5 IN XENOPUS EMBRYOS INDUCES THE FORMATION OF ENLARGED MYOTOMES THROUGH RECRUITMENT OF CELLS OF NONSOMITIC LINEAGE

The myogenic regulatory factors (MRFs) MyoD and Myf5 are the earliest described muscle-specific genes to be expressed in Xenopus development . To study the in vivo effects of overexpressing Xenopus MyoD and Myf5 , synthetic RNAs were microinjected into single blastomeres of 2- to 3 2-cell stage Xenopus embryos. In vivo overexpression of these MRFs ini tiates the precocious and ectopic expression of actin and myosin. The effects of unilateral injection of either mRNA mere indistinguishable; embryos injected at the 2-cell stage showed ipsilaterally enlarged cr anial and anterior trunk myotomes composed of increased numbers of pri mary myotome myocytes. In addition, formation of ectopic muscle in lat eral plate and neural tissue was observed. The MRF-induced effects per sist through secondary myogenesis, with the enlarged cranial myotomes failing to undergo the normal program of degeneration. Experiments com bining MRF RNA and lineage tracer injections showed that myotomal enla rgement is due in part to the contribution of cells of nonsomitic line age to the myotome, rather than to an increase in muscle precursor cel l division. Overexpression of XMyoD and XMyf5 also affected the morpho genesis of the skin and the nervous system. These results reveal that overexpression of XMyoD or XMyf5 in. vivo clearly influences the regul ation of early myogenesis and the morphogenesis of skin and nervous ti ssue. (C) 1994 Academic Press, Inc.